What are the differences between non-time-limited certificates and time-limited certificates?

What are the differences between non-time-limited certificates and time-limited certificates?

Between 1933 and 1990, certificates in dermatology issued by the ABD had no expiration date. These certificates will not expire as long as the diplomate maintains a full, valid, unrestricted license to practice medicine. Time-limited certificates, which began in 1991, require engagement in a continuing certification program.

Diplomates with non-time-limited certificates are encouraged, but not required, to participate in continuing certification activities.

Diplomates with non-time-limited certificates who earn subspecialty certification after 1991 are required to complete continuing certification activities in their subspecialty.